Haptic controller for road vehicles

ABSTRACT

An electric assisted steering system for a motor driven road vehicle, having assist torque signal generating means which generates an assist torque signal for the steering system in response to the driver&#39;s applied torque and sensed vehicle speed to reduce the driver&#39;s steering effort. A yaw rate haptic torque is generated which is based upon vehicle rate error and is arranged to be added to the torque assist signal such that, when the yaw rate error builds up corresponding to increasing steering instability (e.g. understeer or oversteer) of the vehicle, the haptic torque added to the torque assist signal reduces the effective road reaction feedback sensed by the driver in advance of any actual vehicle stability loss whereby to allow the driver to correct appropriately in good time before terminal steering instability is reached. Alternatively, the haptic torque can be based upon vehicle lateral acceleration which is arranged to be subtracted from the torque assist signal such that when vehicle lateral acceleration builds up, corresponding to tighter cornering of the vehicle, the haptic torque subtracted from the torque assist signal increases the effective road reaction feedback sensed by the driver corresponding to the increase in cornering forces generated by the tyres of the vehicle.

DESCRIPTION

[0001] The present invention relates to electric assisted steeringsystems (EAS) in motor driven road vehicles and is concerned inparticular with a control system in a road vehicle adapted to providesteering torque compensation or haptic torque based on the measuredvehicle dynamics, such as yaw rate or lateral acceleration.

[0002] Electric assist steering systems are well known in the art.Electric assist steering systems that use, for example, a rack andpinion gear set to couple the steering column to the steered axle,provide power assist by using an electric motor to either apply rotaryforce to a steering shaft connected to a pinion gear, or apply linearforce to a steering member having the rack teeth thereon. The electricmotor in such systems is typically controlled in response to (a) adriver's applied torque to the vehicle steering wheel, and (b) sensedvehicle speed.

[0003] Other known electric assist steering systems includeelectro-hydraulic systems in which the power assist is provided byhydraulic means under at least partial control of an electrical orelectronic control system.

[0004] There is a desire, at least in certain vehicle market segments,to provide the driver with information about the dynamic state of thevehicle via the steering wheel torque. The effects that are most commonare an increase in driver torque as the lateral acceleration on thevehicle increases (the handwheel seems to become heavier), and a suddendrop in driver torque (the handwheel seems to become much lighter) whenthe vehicle reaches ternl understeer. (Terminal understeer is consideredto be when an increase in handwheel angle, no longer gives an increasein vehicle yaw rate.) Traditionally these effects have been produced bycarefwl design of the steering system, but modern power assistedsteering systems, and space and other compromises in the design ofsteering systems, has lead to the effects becoming much less noticeable.However, there is a general perception that these effects improve thehandling of a vehicle, and therefore they can be quite important incertain market segments.

[0005] It is an object of the present invention to improve the hapticinformation that the driver receives from the steering system and toprovide an algorithm using information already contained in the carabout the dynamic state of the car, to artificially recreate thesteering feel properties described above.

[0006] In its broadest scope, the invention is concerned with a controlalgorithm that uses signals from motion sensors to adjust the assisttorque provided by a power assisted steering system in such a way thatinformation about the dynamic state can be gleaned from the feel of thesteering. In one embodiment, the algorithm recreates the increase indriver(handwheel) torque felt by a driver in proportion to lateralacceleration, and in another embodiment the decrease in driver torquewhen the vehicle enters termninal understeer.

[0007] In accordance with a first aspect of the present invention, thereis provided a power assisted steering system for a motor driven roadvehicle, the system including assist torque signal generating meansarranged to generate an assist torque signal for the steering system inresponse to the driver's applied torque and sensed vehicle speed andeffective to reduce the driver's steering effort and a means forgenerating a haptic torque based upon vehicle yaw rate error which isarranged to be added to the torque assist signal such that when the yawrate error builds up, corresponding to increasing steering instabilityof the vehicle, the haptic torque added to the torque assist signalreduces the effective road reaction feedback sensed by the driver inadvance of any actual vehicle stability loss whereby to allow the driverto correct appropriately in good time before terminal steeringinstability is reached.

[0008] Such a system has the advantage of drawing steering instabilityconditions (e.g. understeer or oversteer) to the attention of thedriver.

[0009] Preferably, the assist torque signal generating means comprisesan electric motor.

[0010] Yaw rate error can be established by comparing an estimated yawrate derived from measured values of steering angle and vehiclelongitudinal velocity, with measured vehicle yaw rate.

[0011] Preferably, the yaw rate error is saturated, if necessary, toprevent excessive demand and scaled by a gain map.

[0012] The gain is preferably controlled in accordance with yaw rateerror, such that a low yaw rate error results in a relatively low gainand a high yaw rate error results in a relatively large gain so as toincrease the assist torque from the power steering and make the steeringfeel light to the driver.

[0013] In some embodiments, a plurality of gain maps are provided, themost suitable to comply with the prevailing conditions being arranged tobe selected automatically from a judgement of road surface conditionsbased on measured data, such as measured yaw rate error and columntorque.

[0014] The haptic torque is preferably established by scaling thesteering column torque using the scaled yaw rate error, the haptictorque being added to the torque assist to provide an output for drivingthe electric motor.

[0015] A dynamic yaw rate error signal can be derived from a dynamic yawrate estimation; a functionally equivalent lateral acceleration errorsignal can also be derived from an equivalent dynamic lateralacceleration estimator.

[0016] Thus, in accordance with a second aspect of the presentinvention, there is provided a power assisted steering system for amotor driven vehicle, the system including assist torque signalgenerating means aranged to generate an assist torque signal for thesteering system in response to the driver's applied torque and sensedvehicle speed and effective to decrease the driver's steering effort,and a means for generating a haptic torque based on vehicle lateralacceleration which is arranged to be subtracted from the torque assistsignal such that when vehicle lateral acceleration builds up,corresponding to tighter cornering of the vehicle, the haptic torquesubtracted from the torque assist signal increases the effective roadreaction feedback sensed by the driver corresponding to the increase incornering forces generated by the tyres of the vehicle.

[0017] This latter arrangement enables the controller to be furthertuned to give a heavier steering feel up to the point of understeerfollowed by a lowering in column torque once impending understeer hasbeen determined.

[0018] In accordance with a third aspect of the present invention thereis provided a power assisted steering system for a motor driven roadvehicle, the system including assist torque signal generating meansarranged to generate an assist torque signal for the steering system inresponse to the driver's applied torque and sensed vehicle speed andeffective to reduce the driver's steering effort, and a means forgenerating a haptic torque based upon vehicle lateral acceleration errorwhich is arranged to be added to the torque assist signal such that whenthe lateral acceleration error builds up, corresponding to increasingsteering instability of the vehicle, the haptic torque added to thetorque assist signal reduces the effective road reaction feedback sensedby the driver in advance of any actual vehicle stability loss whereby toallow the driver to correct appropriately in good time before terminalsteering instability is reached.

[0019] In accordance with a fourth aspect of the present invention,there is provided a power assisted steering system for a motor drivenroad vehicle, the system including assist torque signal generating meansamnwged to generate an assist torque signal for the steering system inresponse to the driver's applied torque and sensed vehicle speed andeffective to reduce the driver's steering effort, and a means forgenerating a haptic torque based either upon (a) vehicle yaw rate erroror (b) lateral acceleration error which is arranged to be added to thetorque assist signal such that when said error builds up, correspondingto increasing steering instability of the vehicle, the haptic torqueadded to the torque assist signal reduces the effective road reactionfeedback sensed by the driver in advance of any actual vehicle stabilityloss whereby to allow the driver to correct appropriately in good timebefore terminal steering instability is reached.

[0020] In the latter case, therefore, the overall control method is thesame for providing the haptic torque but that the pre-processing part ofthe method optionally takes either yaw rate or lateral acceleration asthe controlling input and where the use of lateral accelerationadditionally provides further benefits in providing a haptic torque upto the point of impending understeer.

[0021] The invention is described further hereinafter, by way of exampleonly, with reference to the accompanying drawings, in which:

[0022]FIG. 1 is a block diagram illusrating one embodiment of a controlsystem in accordance with the present invention;

[0023]FIG. 2 shows examples of controller gain maps that can be used inthe present invention;

[0024]FIG. 3 illustrates a derivative fiuntion of the steady statecontroller of FIG. 1;

[0025]FIG. 4 substitutes lateral acceleration for yaw rate;

[0026]FIG. 5 illustrates the provision of a modifying torque based onlateral acceleration;

[0027]FIG. 6 is a block diagram illustrating a further embodiment of acontrol system in accordance with the present invention;

[0028]FIG. 7 is a block diagram illustrating a fiurter embodiment of acontrol system in accordance with the present invention;

[0029]FIG. 8 illustrates yaw rate estimation and reverse detection;

[0030]FIG. 9 is a block diagram of a yaw rate estimator;

[0031]FIG. 10 is a block diagram of a lateral acceleration estinator;

[0032]FIG. 11 is a block diagram illustrating reverse detection;

[0033]FIG. 12 is a block diagram illustrating understeer control;

[0034]FIG. 13 illustrates a yaw rate error to driver torque scalinglook-up table;

[0035]FIG. 14 is a block diagram of a torque control section;

[0036]FIG. 15 illustrates scaling correction shape;

[0037]FIG. 16 illustrates lateral acceleration feel control; and

[0038]FIG. 17 is a block diagram of a fuirther embodiment similar toFIG. 7 but with the yaw rate estimator replaced by a lateralacceleration estimator.

[0039] Referring first to FIG. 1, the vehicle steer angle andlongitudinal velocity are input to element 10 where an estimate isestablished of the yaw rate demanded by the driver of the vehicle. Theyaw rate estimation is based for example on the steady state understeerequation, expressed as:$\hat{r} = {\frac{V_{x}}{l\quad \left( {1 + \left( {V_{x}/V_{c\quad h}} \right)^{2}} \right)}\frac{\delta_{sw}}{G_{s}}}$

[0040] where V_(X) is the vehicle longitudinal velocity, l is the wheelbase, V_(ch) is the vehicle characteristic speed, δ_(SW) is thehandwheel angle and G_(S) is the gain of the steering system from roadwheels to handwheel. This estimated value is then passed through a firstorder low pass filter, tuned to give the estimate similar lag to thevehicle. Careful selection of the break point in this filter allows thepoint where the steering goes light in relation to the loss of steerauthority to be controlled.

[0041] The resulting estimated yaw rate is compared at 12 with a signalrepresentative of the actual vehicle yaw rate, as measured by a VehicleStability Controller (VSC) or similar sensor, to generate a yaw rateerror signal on line 14. The yaw rate error is then saturated at 16 toprevent excessive demand and scaled by a gain map 18.

[0042] The saturation block 16 prevents the yaw rate error from reachingtoo high a level. Experience has shown that if the yaw error is allowedto increase too much, then this can excite an instability in the EASsystem. This may be dependent to some extent on the particularcharacteristics of the EAS system fitted to the vehicle, but thesaturation also prevents the system producing excessive torques in theevent of an error. It may be necessary sometimes to tune the value ofthis saturation in dependence upon the surface that the vehicle is on. Apossibility is to use column torque and yaw rate error as indices into alook-up table.

[0043] The gain at 18 is varied in accordance with the yaw rate error. Alow yaw rate error indicates the linear regime referred to above inwhich the vehicle is operating at constant forward speed and beforeterminal understeer is reached, and where therefore a low gain isrequired. On the other hand, a high yaw rate error is indicative ofexcessive understeer, and therefore a large gain is required to increasethe assist torque from the power steering and make the steering feellight.

[0044] The haptic torque on line 20 is established by using the scaledyaw rate error signal from the gain map 18 to scale at 28, the columntorque (Tcol) which has been low pass filtered at 22 to prevent excitingunstable modes in the power steering. By scaling the column torque at28, rather than adding to it, the controller is prevented from enteringa region where it may attempt to drive the steering system against thedriver. In this manner, the inherent self-centring of the steering ismaintained. If the driver releases the steering wheel, then the columntorque falls to zero, and the haptic torque also falls to zero.

[0045] The output fromrthe controller is thus determined by multiplyingat 28 the column torque (Tcol) with the output from the gain map 18. Asdescribed above, the column torque is low pass filtered at 22, again toprevent excitation of the EAS unstable modes. Careful design of thisfilter 22 may show that is possible to guarantee stability and allow theremoval of the saturation element 16. The result of the multiplicationat 28 is then added to the assist torque (Tassist) generated from thepower steering controller, and fed to the power steering (EAS) motor.

[0046] The “Abs” blocks 24 and 26 in FIG. 1 are included so that theabsolute values, or magnitudes, of the input signals are taken.

[0047] The yaw rate error in the arrangement of FIG. 1 is scaled by again map at 18 in establishing the final output from the controller. Thegain value is dependent on the value of the yaw rate error and also onthe characteristics of the surface on which the vehicle is running, ie.high Mu or low Mu.

[0048] An example of a controller gain map which can be used is shown inFIG. 2. For low yaw rate error, the scaling is negative, thereforereducing the assist torque and making the steering feel slightlyheavier. The aim is to produce a torque that increases with handwheelangle. For high yaw rate errors, the gain is much higher, giving a largepositive output that greatly increases the assist torque and makes thehandwheel feel light. The shape of these maps can be varied to producethe desired feel in the steering system.

[0049] A single map can be used or, preferably, a plurality of maps canbe available, the most suitable of which to suit the prevailingcircumstances can be selected automatically from a judgement ofroadsurface conditions based, for example, on the measured yaw moment errorand column torque, or on tyre slip or video detection routines.

[0050] Improvement in the haptic information that the driver receives isprovided in the above described system by altering the torque in thesteering column in two ways. In the broadly linear operating region atconstant forward speed and before terminal understeer is reached, thetorque in the steering column is gradually increased as the handwheelangle is increased. This provides the driver with a haptic indicationvia the handwheel of the amount of lateral acceleration on the vehicle.When terminal understeer is reached such that additional handwheel anglefails to increase the vehicle yaw rate, the torque in the steeringcolumn is greatly reduced. This causes the handwheel to become verylight, providing the driver with an indication that the limit oftraction has been reached. Turning of the controller allows this drop intorque to happen slightly ahead of the actual loss of traction,providing the driver with a short but usable response time before steerauthority is lost.

[0051] Traditional power steering systems attempt to control the torqueapplied by the driver to within limits. This can easily lead to a systemwhere there are none of the haptic features described above and may havelittle or no change in torque with vehicle dynamic state. The design ofthe steering geometry also has a significant effect on the feel of thesteering. If the geometry is such that there is no build up of steeringtorque, or drop in torque once the limit is reached, then no simplepower steering system will be able to put that feel back. The presentsystem considers what yaw rate the driver is demanding and the actualyaw rate of the vehicle to determine what the torque in the steeringsystem should be. The assist torque generated by the power steeringsystem is then adjusted accordingly. The proposed system thereforeproduces steering feel which is independent of the power steering systemand the steering geometry.

[0052] Referring now to FIG. 3 there is shown a further development ofthe first aspect of the invention illustrated in FIG. 1, essentiallybeing a derivative function of the steady state controller of FIG. 1 andproviding dynamic understeer haptic torque based on yaw rate estimation.In the controller of FIG. 3, use is again made of steer angle andlongitudinal velocity as in the first aspect but there is furtherincluded a dynamic component, in this case yaw rae. The resulting outputprovides a yaw rate error which when corrected or scaled against drivertorque provides a haptic torque, which is optionally limited, foraddition to the power assist torque generated by the power assistsystem.

[0053] Refering next to FIG. 4 there is shown an altenative development,similar to FIG. 3, but which substitutes lateral acceleration for yawrate, thereby providing dynamic understeer haptic torque based onlateral acceleration estimation. In the controller of FIG. 4, use isagain made of steer angle and longitudinal velocity but there isincluded the further dynamic component of lateral acceleration insteadof yaw rate. The resulting output provides a lateral acceleration errorwhich when corrected or sealed against driver torque provides a haptictorque, which is optionally limited, for addition to the power assisttorque generated by the power assist system.

[0054]FIG. 5 illustrates the provision of a modifying torque based uponlateral acceleration to give a haptic response as lateral acceleration(and hence cornering force) increases up to the point of impendingundersteer.

[0055]FIG. 6 illustrates an arrangement for providing haptic torquebased on any of the arrangements of FIGS. 1, 3 or 4 and includinglateral acceleration feedback as shown in FIG. 5. This illustrates theoverall system where a haptic torque based upon lateral acceleration canbe used to provide a dynamic response up to the point of impendingundersteer and where a dynamic fiuction, such as yaw rate or lateralacceleration can be fuirther added to provide an additional response atthe point of impending understeer.

[0056] In the embodiment of FIG. 7, the dynamic finction is yaw rate andthe dynamic error is therefore yaw rate error. The yaw rate estimator 30uses vehicle state information to calculate the yaw rate that the driveris demanding based on the assumption that the vehicle is linear. This isthen passed to the understeer control 32, which calculates a correctionto the assist torque being generated by the steering system. Inaddition, the lateral acceleration block 34 uses a lateral accelerationsignal to calculate a separate torque correction signal. The differencebetween these two correction signals at 36 becomes the Haptic (feel)torque which is added to the steering system assist torque at 38.

[0057] Yaw Rate Estimator and Reverse Detection

[0058] There are a number of ways in which the yaw rate or lateralacceleration of the vehicle can be estimated. Steady state estimation isthe simplest, but has been found to be unreliable at low vehicle speeds.Dynamic estimation is more complicated, but to date, no insurmountableproblems have been encountered. The final solution is to use informationfrom within the VSC. One further possible solution is to use lateralacceleration instead of yaw rate, as described in the last paragraph. Atthe same time as the yaw rate or lateral acceleration is beingestimated, a check is made to detect whether the vehicle is reversing.If the vehicle is reversing, then the yaw rate error or lateralacceleration error is set to zero. This prevents the understeercontroller affecting the steering feel while the vehicle is reversingwhich may be confusing for the driver. Yaw rate estimation and reversedetection is illustrated in FIG. 8.

[0059] Yaw Rate Estimator—Steady State Estimation

[0060] As described hereinbefore, yaw rate estimation can be based onthe steady state understeer equation;$\hat{r} = {\frac{V_{x}}{l\quad \left( {1 + \left( {V_{x}/V_{c\quad h}} \right)^{2}} \right)}\frac{\delta_{sw}}{G_{s}}}$

[0061] where V_(X) is the vehicle longitudinal velocity, l is the wheelbase, V_(ch) is the vehicle characteristic speed, δ_(SW) is thehandwheel angle and G_(S) is the gain of the steering system from roadwheels to handwheel. This estimated value is then passed through a firstorder low pass filter, tuned to give the estimate similar lag to thevehicle. Careful selection of the break point in this filter allows thepoint where the steering goes light in relation to the loss of steerauthority to be controlled.

[0062] Experience with this method has highlighted a number ofundesirable features. The estimator breaks down at low speed causing thecontroller to activate in “parking” type manoeuvres where it is highlyundesirable. Also the performance of the estimator does not reflectchanges in the vehicle performance as the speed varies.

[0063] Dynamic Yaw Rate Estimator

[0064] The yaw rate estimator can be improved by using a full orderobserver based on the bicycle model with yaw rate feedback, see FIG. 9.The bicycle model is written as: $\begin{matrix}{{\frac{\quad}{t}\begin{bmatrix}V_{y} \\r\end{bmatrix}} = {{\underset{\underset{A}{}}{\begin{bmatrix}\frac{- \left( {C_{af} + C_{a\quad r}} \right)}{m\quad V} & {\frac{- \left( {{a\quad C_{af}} - {b\quad C_{ar}}} \right)}{m\quad V} - V} \\\frac{- \left( {{a\quad C_{af}} - {b\quad C_{ar}}} \right)}{I_{zz}\quad V} & \frac{- \left( {{a^{2}\quad C_{af}} + {b^{2}C_{ar}}} \right)}{I_{zz}\quad V}\end{bmatrix}}\begin{bmatrix}V_{y} \\r\end{bmatrix}} + {\underset{\underset{B}{}}{\begin{bmatrix}\frac{C_{af}}{m} \\\frac{C_{ar}}{I_{zz}}\end{bmatrix}}{\delta (t)}}}} & (1) \\{r_{meas} = {\underset{\underset{C}{}}{\begin{bmatrix}0 & 1\end{bmatrix}}\begin{bmatrix}V_{y} \\r\end{bmatrix}}} & (2)\end{matrix}$

[0065] The closed loop form of the estimator is written as:$\begin{matrix}{{\frac{\quad}{t}\begin{bmatrix}{\hat{V}}_{y} \\\hat{r}\end{bmatrix}} = {{\left\lbrack {A - {GC}} \right\rbrack \begin{bmatrix}{\hat{V}}_{y} \\\hat{r}\end{bmatrix}} + {B\quad {\delta (t)}} + {G\quad r_{meas}}}} & (3)\end{matrix}$

[0066] with G selected to place the poles of the estimator at locations10 times faster than the open loop model. At first this seemsunnecessary as the actual yaw rate of the vehicle is being used in anestimator to determine the yaw rate of the vehicle. It is true that atlow yaw rates, the estimated and measured values will coincide. However,the estimator is based upon an entirely linear model, which becomes lessaccurate as the vehicle becomes more non-linear during increasingundersteer. Hence when the vehicle starts to understeer, the estimatedand actual yaw rates start to diverge causing an error that is used tochange the assist torque value. The use of yaw rate feedback thereforereduces the susceptibility of the estimator to changes in dynamics withspeed.

[0067] Experimentation has also shown that the estimator is robust toparameter change though this can be flirther improved by the use oflateral acceleration as an additional feedback signal.

[0068] Lateral Acceleration Estimationn

[0069]FIG. 10 is a diagram corresponding to FIG. 9 but illusing the casewhere the estimator is a lateral acceleration estimator block Thecontroller is substantially the same as for the yaw rate estimtor butwith the addition of the term D′ from the lateral acceleration equationbelow.

[0070] The system is the same as for the dynamic estimator, only theinputs to the observer are steer angle and lateral acceleration (insteadof yaw rate), and the output is an estimate of lateral acceleration,Equation 4. By subtracting the estimated lateral acceleration from theactual lateral acceleration, an error signal is generated that can beused in the same way as the yaw rate error signal of the firstembodiment of FIG. 1 $\begin{matrix}{a_{y{({meas})}} = {{\underset{\underset{C^{\prime}}{}}{\begin{bmatrix}\frac{- \left( {C_{af} + C_{a\quad r}} \right)}{m\quad V} & \frac{- \left( {{a\quad C_{af}} - {b\quad C_{ar}}} \right)}{m\quad V}\end{bmatrix}}\begin{bmatrix}V_{y} \\r\end{bmatrix}}\underset{\underset{D^{\prime}}{}}{\left\lbrack \frac{C_{af}}{m} \right\rbrack}{\delta (t)}}} & (4)\end{matrix}$

[0071] The state equation for the lateral acceleration observer is givenby: $\begin{matrix}{{\frac{\quad}{t}\begin{bmatrix}{\hat{V}}_{y} \\\hat{r}\end{bmatrix}} = {{{\left\lbrack {A - {G^{\prime}C^{\prime}}} \right\rbrack \begin{bmatrix}{\hat{V}}_{y} \\\hat{r}\end{bmatrix}}\quad \overset{\Cap}{x}} + {\left\lbrack {B - \quad {G^{\prime}D^{\prime}}} \right\rbrack {\delta (t)}} + {{G\quad}^{\prime}a_{y{({meas})}}}}} & (5)\end{matrix}$

[0072] Reverse Detection

[0073] A preferred reverse detection algorithm is depicted in FIG. 11.The reverse flag is zero, indicating that the vehicle is reversing, wheneither yaw rate is in the deadzone, or they have different signs. Thedeadzones 40, 42 limit the effect of noise on the yaw rate signals. Theyare set such that for a stationary vehicle, the noise on the yaw ratedoes not exceed the deadzone. While the signal is in the deadzone, thenthe output of the block is zero. The sign blocks 44, 46 take the sign ofthe input signal. If the input signal is zero, then the sign value isalso zero, otherwise it is plus if the input is positive or minus one ifthe input signal is negative. The saturation 48 is set so that if theinput is 1 or greater, then the output is 1. If the input is zero orless, then the output is zero. This limits the reverse flag to 0 or 1.Reverse detection could equally well be achieved by comparison ofestimated and actual lateral acceleration.

[0074] The yaw rate error, i.e., the difference between the yaw ratethat the driver is demanding and the actual yaw rate of the vehicle, isfound by subtracting the actual vehicle yaw rate from the estimated yawrate as indicated in FIG. 8 at 50.

[0075] The Driver Torque Limit block limits the value of driver torqueto pre-defined levels. As the amount of torque that the understeercontrol injects into the system is proportional to the driver torque,limits on the driver torque effectively place limits on the anount oftorque that the system can inject.

[0076] Understeer Control

[0077] The understeer control comprises a number of sections asindicated in FIG. 12. The yaw rate error to scaling map 52 controls thesteering feel, while the torque control 54 prevents excessive udersteertorque, cancelling the assistance torque from the power steering, andeven reversing the torque applied by the driver. Again, the controlstructure of FIG. 12 could equally well produce an understeer torquebased upon a lateral acceleration error estimate.

[0078] Yaw Rate Error to Scaling Map

[0079] The input yaw rate error is used as the index input into alook-up table. The output of this look-up is the driver torque scaling.The elements of the look-up table are low for small yaw rate errors andlarge for high yaw rate errors as indicated in FIG. 13 (Yaw Rate Errorto Driver Torque Scaling Look-up Table). Again, the principles of FIG.13 could be applied equally to lateral acceleration error.

[0080] By varying the elements in the look-up table, the effect of thecontroller can be tuned to different vehicles and to give differentperformances. By making the transition from high to low occur at loweryaw rate errors, the steering feel can be made to go light well beforethe car reaches terminal understeer. If the torque scaling gain is madeslightly negative initially, then the steering can be made to feelheavier at low yaw rates. This is very similar to an increase in drivertorque in proportion to lateral acceleration.

[0081] Torque Control

[0082] It is possible for the understeer controller to generate a torquethat is larger than the assist torque being generated by the powersteering system. If this occurred, the torque felt by the driver wouldbe reversed. Not only would this be uncomfortable and confuising, therewould be the potential that the steering would wind on more lock insteadof returning to straight ahead when the driver removed his hands. Toprevent this torque reversal, the algorithm shown in FIG. 14 (blockdiagram of torque control section) has been constructed. This attemptsto keep the driver torque above a predefined torque threshold which isachieved by comparing at 56 the actual driver torque with the torquethreshold and generating a scaling between 0 and 1. By applying thisscaling to the output of the understeer controller, the effects of theundersteer controller can be controlled.

[0083] The algorithm is designed such that a scaling correction value isproduced with the shape shown in FIG. 15 where T_(threshold) is thetorque value below which the driver torque should not fall and k_(tune)is the value of tuning gain.

[0084] The scaling correction has 3 stages. In the first stage, thedriver torque is very low and it is essential to quickly limit theeffect of the understeer controller to prevent torque reversal. This isachieved by setting the scaling correction to zero, thereby nullifyingthe effect of the understeer controller. This is in effect a deadzoneand is useful in dealing with noise from the torque sensor. The secondstage is between some designed point and the torque threshold value. Inthis section, the output from the torque controller ramps linearly from0 to 1 producing smooth control of the understeer controller output. Thelast section is where the driver torque is above the torque threshold.The scaling correction is one, therefore having no effect on theundersteer controller output.

[0085] Torque Threshold

[0086] This block 58 is a constant equal to the level at which is itdesired that te driver torque does not fall below. A satisfactory valuehat been found to be 1 Nm.

[0087] Max

[0088] The MAX blocks 60, 62 simply take the maximum value of their twoinputs. In both cases they prevent the input failing below zero whichcould cause a negative scaling correction. This would provide a torquereversal at the driver, which would be highly undesirable as it wouldconfusing for the driver.

[0089] Tuning Gain & Filter

[0090] The tuning gain and filter block 64 smooths the driver torquesignal so that smoother control with less switching is produced. Thetiming gain is used to control the size of the deadband as described by:$T_{deadbond} = \frac{{T_{threshold}k_{tune}} - 1}{k_{tune}}$

[0091] Lateral Acceleration Control

[0092] The lateral acceleration feel control, FIG. 16, is designed toproduce a build up of torque in the steering system in proportion to thelateral acceleration of the vehicle.

[0093] Lateral Acceleration Compensation (66)

[0094] The lateral acceleration value supplied by a lateral accelerationsensor is passed through a compensation algorithm. This may simply be afilter to remove noise, or it may be a correction for the lateralacceleration sensor position. The best results are obtained if thelateral acceleration at the front axle is used. As the sensor isunlikely to be placed there, then the compensation element can be usedto compensate for physical displacement of the sensor from a positionmid way between the front wheels where the measured lateral accelerationwould preferably be measured.

[0095] ABS (68)

[0096] The ABS block takes only the magnitude, thereby ensuring that thescaling is always positive and the lateral acceleration torque alwayshas the same sign as the driver torque. If the relative signs were tochange, then the steering feel would be confusing.

[0097] Look-up Table (70)

[0098] The gain that is applied to that lateral acceleration isdependent on speed and is determined in the look-up table. At lowspeeds, the gain is zero, between around 10 and 40 kph the gain rises toa peak value and then remains at there at all higher speeds. A typicalpeak gain value is around 0.3.

[0099] Haptic Torque Limit

[0100] This is a saturation that prevents the haptic torque exceedingpredefined limits. If it does exceed these limits, then it is simplyheld at the limit.

[0101] The major features of a preferred form of the Haptic Controllercan be summarised as follows.

[0102] Torque Change in Understeer can be Artificially Introduced

[0103] If the steering system has been designed such that there isinsufficient driver torque drop off when the vehicle is in understeer,then this can be added using information from other sensors on thevehicle.

[0104] Torque Build up in Proportion to Lateral Acceleration can beArtificially Introduced

[0105] If the steering system has been designed such that there isinsufficient driver torque build up in proportion to lateralacceleration, then this can be added using information from othersensors on the vehicle.

[0106] Applied Torque is Proportional to Driver Torque

[0107] By making the torque applied by the controllers proportional todriver torque, passive vehicle performance is maintained. If the driverremoves their hands from the handwheel, the driver torque falls to zero,the haptic controller torque also falls to zero and the handwheelself-centres.

[0108] Point of Application of Understeer Control is Entirely Tuneable

[0109] The point at which the steering becomes light can be isolatedfrom the vehicle. Therefor it is possible to provide for the steeringbecoming light slightly before terminal understeer, thereby providingthe driver with advanced warning of a loss of traction.

[0110] Amount of Torque Drop off in Understeer is Tuneable

[0111] The amount that the torque drops off in understeer is tuneable insoftware. Therefore it is sinple to create a steering system that has atorque drop off characteristic that is suited to the vehicle.

[0112] Amount of Torque Build up in Lateral Acceleration is Tunable

[0113] The amount that the torque builds up in proportion to lateralacceleration is tuneable in software. Therefore it is simple to create asteering system that has a torque drop off characteristic that is suitedto the vehicle, including mapping it with speed to give a suitable lowspeed characteristic.

[0114] Lateral Acceleration can be Used Instead of Yaw Rate

[0115] The yaw rate signal can be substituted for a lateral accelerationsignal, and the system will work in the same way with only a few changesto the estimator equations. This is advantageous as the cost of alateral acceleration sensor is much less than a yaw rate sensor.

[0116] Algorithm Does not Affect Steering Feel When Reversing

[0117] A simple algorithm detects when the vehicle is reversing based onthe relative signs of the yaw rate and the yaw rate estimate. Bypreventing the controller acting when the vehicle is reversing,confuising changes in steering feel are avoided.

[0118] Minimal Additional Hardware

[0119] There are minimal additional hardware requirements beyond thebase vehicle. One or more cheap lateral acceleration sensors may be allthat is required Glossary of symbols Parameter Parameters l Wheel baseV_(ch) Vehicle characteristic speed G

Steering ratio C

Front tyre lateral stiffness (per axle) C

Rear tyre lateral stiffness (per axle) m Vehicle mass l

Vehicle yaw moment of inertia a Longitudinal distance from vehicle cogto front axle b Longitudinal distance from vehicle cog to rear axle ABicycle model state transition matrix (2 × 2 matrix) B Bicycle modelstate input matrix (2 × 1 matrix) C Bicycle model state output matrix GObserver gain matrix (2 × 1 matrix) D′ Alternative bicycle model outputfeedforward matrix (using lateral acceleration as a model output) C′Alternative bicycle model state output matrix (using lateralacceleration as a model output) G′ Alternative observer gain matrix(using lateral acceleration as a model output) Signal Signals

Yaw rate estimate V_(x), V Vehicle longitudinal velocity δ

Steer angle (handwheel) V_(y) Vehicle lateral velocity t Vehicle yawrate l Time δ

Steer angle (road wheel) r_(meas) Measured yaw rate {circumflex over(V)}_(y) Estimated lateral velocity

Estimated vehicle yaw rate a_(meas) Measured lateral acceleration

1. A power assisted steering system for a motor driven road vehicle, the system including assist torque siga generating means arranged to generate an assist torque signal for the steering system im response to the driver's applied torque and sensed vehicle speed and effective to reduce the driver's steering efforts and a means for generating a haptic torque based either upon (a) vehicle yaw rate error or (b) lateral acceleration error which is arranged to be added to the torque assist signal such that when said error builds up, corresponding to increasing steering instability of the vehicle, the haptic torque added to the torque assist signal reduces the effective road reaction feedback sensed by the driver in advance of any actual vehicle stability loss whereby to allow the driver to correct appropriately in good time before terminal steering instability is reached.
 2. A power assisted steering system for a motor driven road vehicle, the system including assist torque signal generating means arranged to generate an assist torque signal for the steering system in response to the driver's applied torque and sensed vehicle speed and effective to reduce the driver's steering effort, and a means for generating a haptic torque based upon vehicle yaw rate error which is arranged to be added to the torque assist signal such that when the yaw rate error builds up, corresponding to increasing steering instability of the vehicle, the haptic torque added to the torque assist signal reduces the effective road reaction feedback sensed by the driver in advance of any actual vehicle stability loss whereby to allow the driver to correct appropriately in good time before terminal steering instability is reached.
 3. A power assisted steering system for a motor driven road vehicle, the system including assist torque signal generating means arranged to generate an assist torque signal for the steering system in response to the driver's applied torque and sensed vehicle speed and effective to reduce the driver's steering effort, and a means for generating a haptic torque based upon vehicle lateral acceleration error which is arranged to be added to the torque assist signal such that when the lateral acceleration error builds up, corresponding to increasing steering instability of the vehicle, the haptic torque added to the torque assist signal reduces the effective road reaction feedback sensed by the driver in advance of any actual vehicle stability loss whereby to allow the driver to coirect appropriately in good time before terminal steering instability is reached.
 4. A steering system as claimed in claim 1, 2 or 3, wherein the assist torque signal generating means comprises an electric motor.
 5. A steering system as claimed in claim 1 or 2, wherein yaw rate error is established by comparing an estimated yaw rate derived from measured values of steering angle and vehicle longitudinal velocity, with measured vehicle yaw rate.
 6. A steering system as claimed in claim 5, wherein the yaw rate error is saturated, if necessary, to prevent excessive demand and scaled by a gain map.
 7. A steering system as claimed in claim 6, wherein the gain is controlled in accordance with yaw rate error, a low yaw rate resulting in a relatively low gain and a high yaw rate resulting in a relatively large gain so as to increase the assist torque from the power steering and make the steering feel light to the driver.
 8. A steering system as claimed in claim 6 or 7, wherein a plurality of gain maps are provided, the most suitable to comply with the prevailing conditions being arranged to be selected automatically from a judgement of road surface conditions based on measured data.
 9. A steering system as claimed in claim 6, 7 or 8, wherein the haptic torque is established by scaling the steering column torque using the scaled yaw rate error, the haptic torque being added to the torque assist to provide an output for driving the electric motor.
 10. A power assisted steering system for a motor driven vehicle, the system including assist torque signal generating means arranged to generate an assist torque signal for the steering system in response to the driver's applied torque and sensed vehicle speed and effective to decrease the driver's steering effort, and a means for generating a haptic torque based on vehicle lateral acceleration which is arranged to be subtracted from the torque assist signal such that when vehicle lateral acceleration builds up, corresponding to tighter cornering of the vehicle, the haptic torque subtracted from the torque assist signal increases the effective road reaction feedback sensed by the driver corresponding to the increase in cornering forces generated by the tyres of the vehicle.
 11. A steering system as claimed in claim 1, 2 or 3, wherein the haptic torque generating means includes a dynamic estimator adapted to produce a dynamic error based on measured values of steer angle, vehicle dynamic rate and vehicle longitudinal velocity.
 12. A steering system as claimed in claim 11, wherein the dynamic estimator is a yaw rate estimator.
 13. A steering system as claimed in claim 12, including a reverse detection device for detecting whether the vehicle is reversing and comprising means for detecting when either the actual yaw rate or the estimated yaw rate lies in a deadzone, or they have different signs.
 14. A steering system as claimed in claim 11, wherein the dynamic estimator is a lateral acceleration estimator.
 15. A steering system as claimed in claim 14, including a reverse detection device for detecting whether the vehicle is reversing and comprising means for detecting when either the actual lateral acceleration or the estimated lateral acceleration lies in a deadzone, or they have different signs.
 16. A steering system as claimed in any of claims 11 to 15, including an understeer control which produces an understeer torque from the dynamic error and a limited value of driver applied torque.
 17. A steering system as claimed in claim 16, wherein the understeer torque provides the haptic torque which is added to the assist torque to drive a motor of the power assist system.
 18. A steering system as claimed in claim 17, wherein the understeer torque is modified by a feedback signal corresponding to a lateral acceleration torque based on measured lateral acceleration of the vehicle and the vehicle longitudinal velocity.
 19. A steering system as claimed in claim 18, wherein the feedback signal corresponding to a lateral acceleration torque is based on measured lateral acceleration of the vehicle and the vehicle longitudinal velocity.
 20. A steering system as claimed in claim 18 or 19, in which the lateral acceleration torque is futher dependent on the driver applied torque on the steering wheel.
 21. A steering system as claimed in claim 18 or 19, wherein the lateral acceleration torque is developed in dependence upon the product of functions dependent upon lateral acceleration and longitudinal velocity.
 22. A steering system as claimed in any of claims 16 to 21, wherein the understeer control includes a torque control providing a scaling correction gain for correcting the dynamic error signal in dependence upon driver applied torque.
 23. A steering system as claimed in claim 22, wherein the torque control is adapted to seek to keep the driver torque above a predefined torque threshold by comparing the actual driver torque with a torque threshold and generating a scaling between 0 and 1, the scaling being applied to the output of the understeer controller. 